The inventions described below relate to a method for coating a board with liquid coating material, in particular for coating a basis board intended for circuit boards with liquid photoresist (photolacquer), as well as a board coating system for performing the method.
In the production of multi-layer circuit boards, copper-laminated inner layers are normally coated with a photosensitive etch resistant. The etch resistant is exposed with an IC-original and following this, the circuit picture is developed. Since ever finer structures are required in circuit construction, it is necessary to apply the etch resistant very thinly and yet with high accuracy in the distribution of layer thickness. As clean an environment as possible is a prerequisite for this, since even the smallest particles as impurities on or in the applied etch-resistant layer can cause defects.
At the present time, there are two principal methods used for coating basis boards intended for use as circuit boards. In one, a dry film is applied to the basis plates by means of a carrier film. Following the exposure of the dry film the carrier film is then removed in order to enable the development of the board structure by means of an etching process. In another method the basis board is coated with liquid photo resistant by means of one or more coating rollers. Currently horizontal coating apparati with horizontally arranged coating rollers are employed for this purpose. Each coating roller rotates partially in a photoresist bath and is therefore uniformly moistened along its entire length with liquid coating material. The layer thickness is set by means of a doctor roller which is in contact with the coating roller and is adjustable with varying contact pressure against the coating roller. Opposite the coating roller a brace is arranged, thereby forming a pass-through gap therebetween. The brace can be formed as counterroller or as a second coating apparatus with horizontally arranged coating roller. Each board to be coated is supplied to the coating apparatus using a conveyer-belt or transport rollers and is gripped by its coating roller(s) and, if necessary, counterroller and is transported through the pass-through gap by these rollers while being coated. Following the coating the then coated board is gripped by a transport gripper and is then further transported to a next processing step, for example to the drying of the photoresist or to the exposure of the coated basis board. In the conventional coating system described for application of liquid photolacquer, disadvantages exist in that after coating of the board no uncoated free boarder exists so that a gripping apparatus of the transport gripper has to partially grip into the applied lacquer, whereby small lacquer flakes, so-called tinsels, can arise following drying of the lacquer upon opening the gripper. These flakes can lead to impurities on the basis board and hence to defects and to unusability of the later circuit board. Furthermore a skewing of the board can occur during the drawing-in of the base board between the rollers, something which is also detrimental to the accuracy of the coating. Since furthermore, the coating roller is at the same time also used for the transport of the basis board, a corresponding contact pressure against the board is required which also influences the coating thickness, so that its exact setting becomes on the whole more difficult. Furthermore a change of the board format causes problems in that the transport system must be configured to various widths of basis boards. This is because it is typically formed out of transport grippers which grip the board on opposite edges. The gripping of the board and its advancing to the rollers makes for further difficulties in that the boards cannot be correctly gripped by the gripping apparati or by the rollers, since these can exhibit warping. This is especially the case when the boards are intended for intermediate circuit layers and are formed very thinly. In production, this can lead to backups and/or to defective products.
The devices and methods described below provide a method as well as a board coating system of the type mentioned at the start, by means of which boards of the most different formats can be coated easily with improved quality and at the same time higher productivity.
According to the inventive method for the coating of a board with liquid coating material, in particular for coating a basis board intended for circuit boards with liquid photo resist, the board, in contact with a coating roller, is transported for coating past the latter by a transport apparatus. The coating roller here is rotated in a vertical orientation and the board, held by the transport apparatus on its upper edge in a vertically hanging position, is passed by the vertically oriented coating roller during coating. In addition to the production of circuit boards the method can also be employed for the production of other coated boards, like table or other furniture boards made of wood, steel or synthetic material as well as for construction and paneling boards.
With this method, boards of the most different formats can be coated error-free in a rapid and exact manner. The maximal board format is restricted only by the length of the coating roller whereas, with this as a starting point, any smaller boards are coatable without adaptation of the associated transport apparatus. This is because the boards can constantly be transported past the rollers at a constant height level, whereby the part of the board extending along the coating roller in its length is arbitrarily variable up to the maximum length of the coating roller. In that the boards, guided by the transport apparatus, are transported past the coating roller, a more exact coating is possible than if transport were to occur by means of the rollers themselves.
In order to achieve an even faster production process, the board in one preferred embodiment of the invention is in its supply to the transport apparatus, simultaneously gripped by the latter along the entire upper edge and/or in the removal from the transport apparatus the board is simultaneously released along its entire upper edge. To this end, the transport apparatus and an automatic board supply/or a board removal apparatus are formed accordingly.
Further, in order to ensure a smooth production process, the board which is to be coated is advantageously supplied to the transport apparatus through a funnel-shaped, tapered introduction shaft, thereby straigtening the upper edge of the board, which edge is to be gripped by the transport apparatus.
In order to further accelerate the production process during coating, following the coating process the board, gripped on its upper edge by the transport apparatus, is advantageously guided through a drying apparatus in a vertically hanging position in order to dry the applied coating material.
Particularly in the production of a circuit board, it is necessary that the photoresist applied to the board is cooled down to a suitable temperature prior to the exposure process. In order to accelerate the production process as a whole, following the coating process the board in the inventive coating method, gripped on its upper edge by the transport apparatus, is guided in a vertically hanging position through a cooling apparatus to cool the applied coating material. For the case that the board is to be coated on both sides, the board in the coating process is simultaneously coated on both sides by two coating apparati, which reduces the coating time.
To prevent the coating material from running off of the board, the coating material to be applied is preferredly set to a predetermined viscosity by cooling and/or heating.
The coating result can be further improved by heating the board which is to be coated prior to the coating process. To this end a heating source, for example an infrared heat radiator, can precede the coating apparatus. Furthermore the coating can be qualitatively improved by cleaning the boards prior to the coating process.
The board coating system for performing the inventive method is in particular intended for the coating of basis plates, themselves intended for circuit boards and multi-layer internal layers, with liquid photoresist. However, it can also be used for the coating of other boards like table or other furniture boards made of wood, steel or synthetic material as well as for construction and paneling boards. It contains a coating apparatus which itself contains a coating roller for applying the coating material to one side of the board and a supply device for suppling the liquid coating material to the coating roller. The board coating system further contains a brace which is arranged opposite the coating apparatus and which, during the coating, acts from the other side of the board against the force applied to the board by the coating roller. The board coating system further contains a transport apparatus by which, during the coating, the board, in contact with the coating roller, is transported through a gap between the coating roller and the brace. The coating apparatus is formed as a vertical coating apparatus with a vertically oriented coating roller and the transport apparatus is formed such that the board, gripped on its upper edge in a vertically hanging manner by the transport apparatus is led past the coating roller in this vertically hanging position to be coated.
Due to the vertical orientation of the coating roller and the hanging transport of the boards throughout the transport apparatus, boards of different formats can be coated without any sort of adjustment of the transport apparatus. Here, the maximal board format is still restricted by the length of the coating roller, however smaller boards can alternatively be coated and without any change of the system. This is because smaller boards are coated by the respective upper roller portion, while the roller portion below this rotates out of contact with the board, hereby the production process is not affected. With this, an increase in productivity is achieved due to reduced standstill times. Furthermore, since the transport system continously, i.e. also during the coating process, guidingly transports the boards, the boards to be coated are guidingly transported past the coating roller so that a more exact coating is made possible. Since furthermore the board is hangingly transported on one edge, it can be transported through the coating apparatus with this edge projecting from the latter, by which a coating of this edge in connection with an engagement of the transport apparatus on a coated portion of the board can be avoided. In this way tinsel can be avoided, ultimately leading to fewer rejection products and higher productivity.
The supply apparatus for supplying the liquid coating material to the coating roller can be formed in the shape of a coating roller or of a spray device, by which the roller can itself be coated in advance before coating the board. Preferably the supply apparatus contains a receiving space in which the liquid coating material, i.e. for example the photolacquer or photoresist is received, and in which the coating roller rotates longitudinally of a circumferential portion. The coating roller thus rests circumferentially partially in a liquid bath which ensures a continuously sufficient supply of coating material.
The coating roller can be driven synchronously with board advance so that between the board which is transported past the coating roller by the transport apparatus and the coating roller only a roller contact is given. The coating roller can, however, be turned faster or slower relative to board advance to adjust the layer thickness so that between the coating roller and the board which is led past, and which rests in contact with the coater roller, a relative movement arises which leads to a stroking application of the coating material onto the board. In this way, a thicker application can be achieved by rotating the coating roller faster than the advance of the board, while a thinner coating thickness can be achieved by a slower rotating of the coating roller with relatively faster board transport. The coating roller can be rotated not only in the direction of the movement of board transport, but also counter to this movement. This latter aspect is advantageous for, for example, achieving highly exact coatings with low layer thickness.
Adjustment of layer thickness preferentially occurs using a doctor roller which rests on and rotates counter to the coating roller. By varying the contact pressure between the doctor- and the coating roller, the amount of fluid outwardly transported by the coating roller, and therefore the depth of the layer applied by the coating roller can be adjusted. In place of the doctor roller one could also use a stripper resting on the coating roller, by which the coating material carried on the coating roller is glidingly stripped to the desired height.
The receiving space into which the liquid coating material is received can be formed by the case vessel in which the coating roller and the doctor roller are circumferentially partially arranged in a vertical position. A longitudinal and frontal seal of the vessel against the rollers can, for example, be accomplished by a lip seal pressed against the latter. According to a preferred embodiment, the board coating system contains the following components: a vertically oriented doctor roller which rests on the coating roller, and which is adjustably arranged with varying contact pressure against the coating roller to adjust the coating thickness, a vertically arranged corotating roller which sealingly rests on the coating roller, and which is arranged at such a distance from the doctor roller that between the corotatory roller, the doctor roller and the coating roller a tri-laterally closed cavity is formed, a sealing roller arranged in contact not only against the doctor roller, but also against the corotatory roller while sealing the cavity sidewise, and a frontal seal, by which the cavity on the underfrontal side of the roller is sealed while forming the receiving space for the liquid coating material.
In that the receiving space is bordered by four rotating rollers, a continuous and thereby gentle mixing of the coating fluid contained in the receiving space takes place. By adjusting the contact pressure existing between the rollers, the required imperviousness of the receiving space can be ensured. The rollers can be made of the most varied metal or synthetic materials. The coating roller and the sealing roller are preferredly formed as rollers coated with a synthetic material, in particular with soft synthetic material or rubber. The roller core is preferredly made of steel. In contrast, the doctor roller and the corotatory roller are preferredly embodied as complete steel rollers. The rollers coated with synthetic material yield slightly at the contact points to the relative to them harder steel rollers so that a sealing effect is achieved. The outer surface of the coating roller can be structured so that a predetermined layer pattern can be applied to the board.
The contact pressure which the respective rollers exert on one another is preferably adjustable. To this end both of the steel rollers in a preferred embodiment are provided with positionally adjustable axes, while the sealing roller and the coating roller are outfitted with fixed axes. For the adjustment of the required contact pressure against the coating roller and the sealing roller, it is especially preferred that the doctor roller and the corotatory roller are repositionably arranged in a direction which is perpendicular to the connection line between the axis of rotation of the sealing roller and the axis of rotation of the coating roller. In this way an identical pressure increase or pressure decrease can be achieved by the steel roller on each of the sealing roller and the coating roller. As explained above, the doctor roller is especially provided variably in its contact pressure against the coating roller in order to set the layer thickness. Since by varying contact pressure against the coating roller the contact pressure against the sealing roller is simultaneously varied, insufficient sealing between the doctor roller and the sealing roller may arise. This can be hindered in that the sealing roller is firmly pressed against the doctor roller in advance in such a manner that the contact pressure is always sufficiently large, even in the case of a reduction as part of an adjustment. It is preferably provided that the sealing roller and the doctor roller each turn on their contact points in the direction of the receiving space so that the liquid coating material is conveyed back into the receiving space by this rotatory motion. In this way sufficient sealing of fluid is ensured, even at a low contact pressure between the doctor and the sealing roller. Furthermore a catching vessel can also be provided under the rollers by which any leakage is caught and returned to the receiving space through conduits. The rollers can have various diameters; however, in order to avoid wear they are preferably formed such that they only roll off of one another. This also reliably prevents the occurence of abrasion particles which could find their way into the receiving space and contaminate the coating material. It is further preferred to form the doctor roller and the corotatory roller of identical diameter so that the coating roller is arbitrarily exchangeable with a freely chooseable diameter.
The fluid level in the receiving space would decrease rapidly due to the continous removal of coating material by the coating roller, thereby complicating a complete and therefore exact coating of the board. The fluid level can be held constant by a constant regulated supply of coating material into the receiving space. The regulation can, for example, be achieved by a sensor arranged in the receiving space, which detects the fluid level, and whose signals are sent back to a supply pump which then itself transports the amount of coating material diverted by the coating roller out of its supply container into the receiving space. According to a preferred embodiment the supply apparatus comprises a supply container for liquid coating material from which coating material is supplied to the receiving space, and an adjustable overflow for setting the fluid level in the receiving space, over which overflow the excess coating material is recycled to the supply container. The overflow forms a simple device for setting the fluid level, which occurs by a corresponding height arrangement of the overflow. Furthermore a continuous circulation of the fluid located in the receiving space can be achieved by the overflow in that the resupplied coating material is retransported into the receiving space from below.
It was determined that the liquid coating material applied to the board, for example the photoresist or the photolacquer, does not run down on the board if it has a viscosity of 500 to 1200 mPa per second and layer thicknesses of 4 to 100 xcexcm, despite the vertical orientation of the coating roller(s). Such running down would otherwise lead to a non-uniform distribution in layer thickness. In order to maintain the viscosity more reliably at these desired values, the receiving space according to a preferred embodiment, is outfitted with a cooling and/or a heating apparatus by which the liquid coating material contained therein can be cooled or warmed for adjustment of its viscosity. Whether a heating or a cooling apparatus, or both together, are employed depends on the coating materials used and the dependence of their viscosity on temperature. For this purpose the cooling and/or heating apparatus is preferably regulated by a thermostat. Another possibility to adjust the viscosity consists of supplying a certain amount of solvent to the liquid coating material depending on the desired viscosity. The desired viscosity of the coating material can then be maintained by a regulated supply of solvent.
Of the four rollers of the coating apparatus, one or more can be driven. Preferably each of all four rollers is simultaneously driven by the driving apparatus, avoiding wear which would otherwise result from the fact that the rollers would drive each through rolling contact. Driving can occur by any type of motor: preferably an electric motor is provided which is connected with the individual rollers by gearing. To this end gears, engaged with one another for driving the rollers, can be connected to each of the rollers. Furthermore the rollers can also be rotatingly connected with one another through a belt or, preferably, through a more exact chain drive.
For large required layer thicknesses and/or low viscosities of the coating material, the coating roller is preferably obliquely arranged with respects to the board to be coated with upwardly increasing separation from the board.
In this way a greater layer thickness is initally achieved in the upper portion of the board, which is then compensated by a slow running of the liquid down the board to the desired uniform layer thickness. Alternatively or additionally to this the coating roller can be patterned on its other surface in such a way that the liquid coating material is transported upwardly on the outer surface of the coating roller by the rotating motion of the latter. This can, for example, be accomplished by a helical chamfer or groove in the outer surface of the coating roller. In order not to affect the layer applied to the board this transport structure is, in contrast to the pattern with which patterned layers are applied to the board, provided in a microscopic size.
Alternatively to the inclined orientation of the coating roller, the doctor roller can also be located upwardly inclined away from the coating roller so that, towards the top, it is pressed with decreasing pressure against the coating roller. In this way more coating material is carried outwardly out of the receiving space by the coating roller and is applied to the board so that the previously described effect is also achieved. The brace can be formed as a brace belt or as a brace bar. However, in order to avoid board damage, particularly of the basis boards intended for circuit boards with thicknesses on the order of tenths of a millimeter, a brace roller is preferably provided as a brace which turns via rolling contact with the advance of the board or is advantageously driven itself.
According to another embodiment a second coating apparatus is provided as brace which is formed in a manner corresponding to the first coating apparatus. In this way the board is simultaneously coatable on both sides.
Although the coating apparatus and the brace can be fixedly arranged, thereby forming an intermediate gap of constant size, the coating apparatus and the brace are preferably adjustably arranged perpendicular to the board, i.e. to its direction of transport. In this way in the production of coated boards, the coating apparatus and the brace can be arranged prior to the coating process in such a separation from one another that a coating of the brace by the coating roller and a subsequent undesired coating of the board by the brace can be avoided. As soon as the board is transported to the coating apparatus from the transport apparatus, the coating apparatus and the brace can be moved towards one another, whereby coating can take place. The contact pressure can be further adjusted by the positional adjustability of the coating apparatus and/or of the brace, representing a further possibility for adjusting the layer thickness.
The transport apparatus can for example be formed as a transport trolley which is moved sideways along a transport path by the coating apparatus. The transport car can be outfitted with a bracket on which a grip apparatus is located for attachment of the board to be transported. The transport apparatus is, however, preferably formed as a transport apparatus lying completely on top which advantageously extends above the system components by which the board is processed, as for example the coating apparatus. Here, the transport apparatus preferably extends out continously from a board supply apparatus to a board removal apparatus.
The location of the transport apparatus laying on top allows monitoring, maintenance and/or cleaning work on the coating apparatus to be performed from the side without hinderance. In this way standstill times of the system are reduced, whereby the productivity of the system is increased.
In order to make more reliably possible that the board is transported to the rollers and to further processing stations in as unwarped a manner as possible, the transport apparatus is preferably formed so that the board is gripped along its entire upper edge by the transport apparatus. By gripping along the upper edge, however, is to be understood not only a continous gripping, but also a pointwise gripping along the entire upper edge which occurs in increments.
In order to more rapidly supply boards to and remove boards from the coating system, an automatic board supply and/or an automatic board removal is preferably provided from which boards are supplied to the transport apparatus and removed from it.
According to an advantageous embodiment, the board supply apparatus comprises a board introduction shaft through which the boards are supplied to the transport apparatus. Towards the top the board introduction shaft is formed in a tapered funnel-shaped manner and discharges immediately adjacent to the gripping apparati of the transport apparatus. By feeding the respective board through the tapered shaft, any shifting or other irregularities of the board on its edge to be gripped by the transport apparatus can be straightened, whereby a transition to the transport apparatus can be ensured free of interruptions. In this way transport backup and rejection products can be avoided.
The transport apparatus can be transformed in such a way that each board, upon hanging on the transport apparatus, is gradually gripped along the upper board edge by the grip apparati according to the zipper principle. However, in order to accelerate the overall production process and thereby increase productivity, the board supply apparatus and the transport apparatus are preferably formed such that the respective board, upon hanging on the transport apparatus, is simultaneously gripped along the entire edge of the board by the transport apparatus. Such is correspondingly preferably provided for the board removal apparatus with respect to the releasing of the board from the transport apparatus.
Trollies on tracks with gripping apparati attached thereto are possible examples of a transport apparatus. A preferred transport apparatus is, however, a circulating transport chain, to whose chain members the gripping apparati are attached. The transport chain represents an economical transport means which in addition is driveable by a pinion in an easily regulated manner. The transport chain is preferably turned around two chain wheels whereas it is normally positioned to be freely moveable in its longitudinal direction. The driven chain wheel and the chain drive, which is preferably provided as an electric motor of regulated rotational speed, are preferably located in the board removal apparatus. Here, the driven chain wheel is provided in a fixedly installed manner, whereas the other turn-around chain wheel is provided in the board supply apparatus in a positionally adjustable manner for maintaining chain tension. The grip apparati can be formed as electronically controlled motorized grippers which, according to the control, open or close. Simple mechanical gripping parts are provided as preferable grip apparati which are biased by spring action in the closing direction and which can be opened by mechanical actuation. The simultaneous opening of multiple such mechanical gripping parts can, for example, be caused by a bar which can simultaneously be pressed against multiple gripping parts located on the transport chain, thereby opening the same. In order to ensure a more rapid production process the board supply apparatus and/or the board removal apparatus, however, comprises an opener chain which is oriented parallel to the transport chain and which circulates with its speed, the opener chain comprising on its links attached force plungers. To open the grip parts, the opener chain with its force plungers is pushed against the grip apparati, whereby the spring-biased grip parts of the grip apparati are opened via a mechanism. In this way boards can be sequentially supplied to and removed from the transport apparatus with a continously circulating transport chain, which reduces the production time for coated boards. Pressing of the opener chain can, for example, be caused by means of electric motor or by means of a pneumatic cylinder.
A drying apparatus for drying the coating material applied to the board by the coating apparatus is preferably provided to accelerate the drying process of the applied lacquer, wherein the board, gripped on its upper edge by the transport apparatus, is transported through the drying apparatus in a vertically hanging position.
It might be required that the board runs through the processing step following the coating with a coating material temperature which is less than the coating material temperature after the coating and/or after the drying. For this reason, according to an embodiment, the board coating system is outfitted with a cooling apparatus for cooling the coating material applied to the board by the coating apparatus, wherein the board, gripped on its upper edge by the transport apparatus, is transported through the cooling apparatus in a vertically hanging position. This is especially the case for basis boards intended as circuit boards which, after coating with photoresist, are normally advanced to the exposure process with photoresist cooled to room temperature.
An optionally provided cleaning of the board is carried out in a cleaning apparatus preceding the coating apparatus, which cleaning apparatus preferably comprises a cleaning roller which is in contact with the board transported past and adhesive roller which rolls off of the cleaning roller, which adhesive roller picks up and holds particles which are possibly picked up from the board by the cleaning roller. For the case that both sides are to be coated, a cleaning apparatus is advantageously provided on both sides of the board. The transport apparatus also preferably extends above the cleaning apparatus so that the latter is also freely accessible from the side without hinderances. The transport apparatus is formed such that the board is transported by it through the cleaning apparatus in a vertically hanging position.